Carrier for cleaning silicon wafers

ABSTRACT

A carrier for receiving and holding a plurality of semiconductor wafers and permitting surfaces of the wafers to receive maximum exposure to ultrasonic waves during a wafer cleaning process in which the carrier and the wafers are immersed in a liquid medium and ultrasonic waves are generated in the liquid medium. The carrier includes two vertical sidewalls and at least three horizontal supporting rods that interconnect the sidewalls and that are collectively positioned for supporting wafers in generally upright, face to face position generally parallel to each other. At least one stabilizing rod extends horizontally between the sidewalls for limiting wafer motion relative to the carrier to steady the wafers. A series of spaced apart teeth on the stabilizing rod define troughs for receiving wafers loosely therein to prevent substantial vibratory deflections of the wafers.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor wafer cleaning, and inparticular to a carrier for holding a plurality of wafers whilemaximizing exposure to ultrasonic waves.

Semiconductor wafers for the microelectronics industry are produced byfirst slicing thin wafers from a silicon crystal ingot. After slicing,the wafers undergo a lapping process to give them a somewhat uniformthickness. The wafers are then etched to remove damage and produce asmooth surface. The final step is polishing the wafers to produce ahighly reflective and damage-free surface on at least one face of eachwafer. The wafers must be cleaned between the lapping and etching stepsto remove contaminants such as lapping grit. If the cleaning process isnot effective, the surfaces of the wafer will be stained with finelapping grit residue. Such residual grit may cause contaminationproblems during electrical device fabrication.

Cleaning of silicon wafers is frequently done by exposing the wafers toultrasonic waves while they are immersed in a liquid medium. First, eachwafer is placed in a cassette or carrier that receives multiple wafersand holds them upright in a parallel, spaced arrangement. The carrierwith the wafers is placed into a cleaning tank where it is immersed in afluid containing a caustic solution, with or without a surfactant toassist in wetting and dispersing dirt. Ultrasonic waves emanating fromwave generators located at the bottom of the cleaning tank areintroduced into the liquid medium. Particles of grit or othercontaminants on the wafer surfaces are removed by exposure to ultrasonicwaves. The carrier is then put into a rinse tank where the wafers arerinsed with deionized water, and finally the wafers are dried withisopropanol vapor.

The carrier must limit vibratory motion of the wafers to prevent theoccurrence of damage. When the carrier is moved, as for example when itis placed into the cleaning tank, the wafers can jostle in the carrieror against each other, forming chips or cracks. To prevent suchbreakage, carriers of the prior art have sometimes had a large structuredesigned for holding wafers in a secure manner. Some of these carriershave multiple slots for receiving and holding peripheral edge regions ofwafers. Each slot extends along a substantial portion of the wafercircumference, as from a position near a bottom upwardly to as high asthe upper ⅓ of the wafer.

A significant problem in the cleaning process is that carriers obstructportions of wafers from being adequately exposed to ultrasonic waves.The carriers of the prior art have a somewhat bulky structure forsupporting the wafers in the slots. The structure blocks or hindersdispersion of ultrasonic waves throughout the liquid medium, therebypreventing the waves from reaching all areas of the wafers withsufficient strength to clean. In particular, upper portions of wafersurfaces are often cleaned inadequately and retain some detrimentalparticle contamination because the waves are blocked before they reachthe upper part of the tank. Therefore, the wafers are cleaned unevenly.Further, the structure of the carrier prevents even drying of the wafersurfaces. Peripheral edge regions of the wafers that are received inslots are not fully dried during the isopropanol drying process, causingwater marks and degraded wafer quality.

SUMMARY OF THE INVENTION

Among the several objects and features of the present invention may benoted the provision of a carrier for receiving and holding a pluralityof semiconductor wafers; the provision of such a carrier for permittingall portions of wafer surfaces to be adequately cleaned during anultrasonic wave irradiation cleaning process; the provision of such acarrier that prevents wafer damage when the carrier is moved orvibrated; the provision of such a carrier that permits even drying ofwafer surfaces; and the provision of such a carrier that is easy to use.

Briefly, a carrier of the present invention receives and holds aplurality of semiconductor wafers and permits surfaces of the wafers toreceive maximum exposure to ultrasonic wave irradiation during a wafercleaning process in which the carrier and the wafers are immersed in aliquid medium and ultrasonic waves are generated in the liquid medium.The carrier is adapted to hold wafers having a size defined by a waferdiameter. The carrier comprises spaced apart sidewalls and horizontalrods collectively positioned for supporting wafers in generally upright,face to face position generally parallel to each other. Each of the rodsextends between and interconnects the sidewalls, the rods beingpositioned in spaced relationship relative to each other so that wafersare exposed to the liquid medium and ultrasonic waves between the rods.The sidewalls are oriented so that the carrier is adapted to hold wafersgenerally parallel to the sidewalls. The sidewalls are sized to have amaximum height that is less than about half of the wafer diameter sothat an upper half of each wafer is exposed to the liquid medium whenthe carrier holding the wafers is immersed in the liquid medium, wherebythe carrier is adapted to hold wafers to permit generally unobstructedpassage of ultrasonic waves through the liquid medium to reach allportions of wafer surfaces with minimal impedance by structure of thecarrier, thereby facilitating effective cleaning.

In another aspect, a carrier of the present invention receives and holdsa plurality of semiconductor wafers and permitting surfaces of thewafers to receive maximum exposure to ultrasonic waves during a wafercleaning process in which the carrier and the wafers are immersed in aliquid medium and ultrasonic waves are generated in the liquid medium.The carrier is adapted to hold wafers having a size defined by a waferdiameter. The carrier comprises two vertical sidewalls in spacedrelationship and at least three horizontal rods positioned in spacedrelation for support of wafers in generally parallel relationship withrespect to each other. Each of the rods extends between the sidewalls. Aseries of spaced apart teeth are on at least one of the rods. Eachadjacent pair of teeth define a channel therebetween having a shapegenerally in the form of a “V” to receive a peripheral edge portion ofthe wafer for holding the wafer in spaced apart position relative toother wafers in the carrier.

Other objects and features of the present invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a carrier for cleaning silicon wafers ofthe present invention;

FIG. 2 is a side elevational view of the carrier;

FIG. 3 is an end elevational view of the carrier;

FIG. 4 is a view in section taken in the plane of line 4—4 of FIG. 1,with a wafer held by the carrier being shown in phantom;

FIG. 5 is a view in section taken in the plane of line 5—5 of FIG. 1;and

FIG. 6 is a view in section taken in the plane of line 6—6 of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and in particular to FIGS. 1 and 4, acarrier for receiving and holding a plurality of semiconductor wafers isindicated generally at 10. The carrier 10 permits surfaces of the wafersW to receive maximum exposure to ultrasonic waves during a wafercleaning process in which the carrier and the wafers are immersed in aliquid medium inside a cleaning tank and ultrasonic waves are generatedinto the liquid medium.

The carrier 10 comprises a frame with two parallel, spaced apartsidewalls 14 and several horizontal rods 16 extending between thesidewalls and oriented perpendicularly to the sidewalls. External walls18 are spaced outside the sidewalls 14 and are parallel to thesidewalls, with one external wall associated with each sidewall. Two endmembers 20 are spaced outside the horizontal rods 16 and are parallel tothe rods, positioned at laterally opposite ends of the carrier. In planview as shown in FIG. 1, the external walls 18 and end members 20 definethe periphery of the carrier structure. All members of the carrier 10have generally rounded corners.

Each sidewall 14 has a maximum height generally in a middle of thesidewall and tapers in height toward ends 22, 24 of the sidewall, asshown in FIG. 2. In the preferred embodiment, an upper edge 26 of eachsidewall has a gentle convex shape that resembles an inverted hull of aboat. The shape provides an efficient support for the structure of thecarrier that is both strong and small in surface area. The two sidewalls14 have generally the same shape and size.

The maximum height H of each sidewall 14, which defines the height ofthe carrier, is sized to range between ¼ and ½ of a diameter of thewafers W that are intended to be held by the carrier. In practice,sidewalls within that height range have demonstrated good carrierstability while retaining generally small surface area. The relativelysmall surface area in comparison to carriers of the prior art ensuresthat an upper half of each wafer is exposed to the liquid medium whenthe carrier holding the wafers is immersed in the liquid medium. Thecarrier 10 permits generally unobstructed passage of ultrasonic wavesthrough the liquid medium to reach all portions of wafer surfaces withminimal impedance by structure of the carrier, thereby to facilitateeffective cleaning.

The length L of each sidewall 14 is sized to be slightly smaller than adiameter of the wafers W held by the carrier. In the preferredembodiment, the sidewall length is at least 90% of the wafer diameterand more preferably 94%. If the sidewall were significantly shorter than90% diameter, the carrier could be unstable, and if it were longer than100% diameter, the carrier is unnecessarily heavy.

The carrier 10 has four legs 30 for supporting the carrier on anunderlying surface and providing stability. Two legs extend from abottom of each sidewall 14 and have equal lengths for supporting thecarrier in a level orientation. The legs 30 are symmetrically arrangedabout a center of the carrier.

The external walls 18 are aligned parallel to the sidewalls 14 alonggenerally an upper half of the carrier, as seen in FIGS. 2 and 3. Theexternal walls 18 are spaced outside the sidewalls 14 and are attachedto the sidewalls by spacer ribs 32. Each external wall 18 has threeopenings 34 therein to minimize weight of the carrier, each openinghaving a simple shape that blends with the carrier contours andallows-for economical manufacture. In the preferred embodiment, theopenings reduce the area of each external wall 18 between 35% and 55%.

An edge periphery 36 of each opening 34 is shaped so that there are noflat, level surfaces along the periphery where liquid could collect andinhibit complete drying. Instead, the edge periphery 36 is tapered inprofile toward a direction away from the carrier so that liquid willflow off the edge, with an angle of taper being between 15° and 60°, andpreferably between 30° and 45°. Other surfaces on the carrier 10,including a top edge periphery of each external wall 18 and of eachsidewall 14, are similarly sloped to inhibit collection or pooling ofliquid.

End members 20 extend between the sidewalls 14 and form handles formanually lifting the carrier or for being gripped and supported by arobot end effector. The end members 20 have rounded bottoms for easygripping and are hollow to lighten the weight of the carrier. Eachsidewall 14 includes a lip 38 located near each end member, as shown inFIG. 2, so that a robot end effector will not slip off the sidewall orend member when lifting the carrier.

The horizontal rods 16 include four supporting rods 40 for holding andsupporting the weight of the wafers and a fifth rod, comprising astabilizing rod 42, as described below. The four supporting rods 40 holdwafers W in generally upright, face to face position with the wafersparallel to each other. Each of the horizontal rods 16 10 extendsbetween and interconnects the sidewalls 14, and may have a shape of acylinder or square pillar or the like. The rods 16 are positioned inspaced relationship relative to each other so that during a wafercleaning process, the wafers are exposed to ultrasonic waves that aregenerated in a bottom of the cleaning tank and propagate between therods. The rods are preferably sized in length so that the carrier canhold between 20 and 30 of the wafers W. Rods of other sizes holding agreater or fewer number of wafers do not depart from the scope of thisinvention.

As shown in FIG. 4, the supporting rods 40 are collectively positionedin spaced relationship defining an arc 44 that corresponds to a segmentof a curved peripheral edge on each wafer W. The rods engage wafersgenerally at point contacts along the peripheral edge. The arc 44extends between the point of contact of the wafer W with the rod 40farthest to the left in FIG. 4 and the point of contact of the waferwith the rod farthest to the right. The arc 44 intersects the other rods40. The arc 44 defined by the rods corresponds to the segment thatcomprises at least about {fraction (2/7)}, and preferably ⅖, of anentire circumference of a wafer W. Preferably, the supporting rods 40define an arc of minimal extent needed to hold the wafers stable duringhandling and ultrasonic cleaning. The supporting rods 40 are shown inthe drawings having a generally symmetrical arrangement with evenspacing about the center of the carrier 10. Other positions of the rods,including non-symmetrical spacings, do not depart from the scope of thisinvention.

First and second rods of the supporting rods 40 extend between locationson each of the two sidewalls 14 near ends 22, 24. Third and fourth rodsextend between locations on the sidewalls 14 spaced generally inwardlyof the ends 22, 24. A series of spaced apart teeth 50 are located on atleast one of the supporting rods 40. In the preferred embodiment, teeth50 are located on the first and second rods, but the third and fourthrods are free of teeth. It is understood that other combinations arepossible, such as only one rod having teeth or all rods. The series ofteeth 50 extend along substantially an entire length of each supportingrod 40 which has teeth. It is understood that rods may have teeth ononly a portion of the extent of the rod without departing from the scopeof this invention.

Adjacent pairs of teeth 50 define V-shaped channels, indicated generallyat 52 in FIG. 5, shaped for receiving peripheral edge portions ofwafers. Each channel 52 is adapted to receive a peripheral edge portionof the wafer W for holding the wafer in spaced apart position relativeto other wafers in the carrier. The teeth 50 have a generally conicalslope or taper with a constant included angle A that is between 35° and50°. A tip 54 of each tooth is preferably blunted so as to facilitateentry of a wafer into the channel 52 and inhibit damage to a siliconwafer that may inadvertently impact the tip.

The fifth horizontal rod comprises the stabilizing rod 42 to limit wafermotion relative to the carrier for steadying of any vibration of wafersand holding wafers in position. The stabilizing rod 42 is perpendicularto the sidewalls 14 and extends between locations near the center ofeach sidewall. It is understood that the stabilizing rod could belocated anywhere along the carrier 10 without departing from the scopeof the invention. The stabilizing rod 42 has a series of spaced apartteeth 56 defining Y-shaped troughs or channels, indicated generally at58 in FIG. 6, between each two adjacent teeth for receiving peripheraledge portions of wafers. Each trough 58 is sized and shaped so that thewafer is loosely received therein. The teeth 56 on the stabilizing rod42 have generally vertical edges 60 forming sides of the troughs, thesides of each trough being in parallel spaced relationship with aspacing S that exceeds a thickness of a wafer received therein. Thesides of each trough are spaced so that S is preferably between about10% and about 20% greater than a thickness of a wafer. Each wafer issuspended above a bottom 62 of the trough 58 and does not touch theteeth 56, unless vibrated or misaligned. The bottom 62 of the trough isflat, although it is not necessarily flat for proper functioning of theinvention. Each trough 58 is shaped to permit the wafer therein to movein vibration between a nominal position in which the wafer is suspendedin the trough and remains substantially free of any contact with theteeth 56 on the stabilizing rod 42, and a deflected position in whichthe wafer engages one tooth on the stabilizing rod at a side 60 of thetrough to prevent further vibratory deflection of the wafer. The teeth56 on the stabilizing rod have a generally conical slope or taper with aconstant included angle B that is between 35° and 50°. A tip 64 of eachtooth is preferably blunted so as to not damage a silicon wafer that mayinadvertently impact the tip.

The carrier 10 is a one-piece molded plastic construction. The materialis a resin that is essentially inert and resistant to effects ofchemicals in the cleaning solution and to effects of high temperatures,thereby to preclude deterioration, warpage, or softening when exposed tochemicals and high temperatures. In the preferred embodiment, thecarrier is made of a fluorine-based resin, such as PFA or PTFE, oranother resin such as PEEK, or PBT.

In operation, a number of silicon wafers W are placed into the carrier10 for cleaning to remove minute dust particles and lapping gritresidue. Each wafer is inserted so that the edge simultaneously engagesfour contact points 66 (FIG. 4) corresponding to the supporting rods 40to support the wafer. These include two points at the first and secondrods where the wafer edge is engaged in V-shaped channels 52 of FIG. 5,and two points at the third and fourth rods which have no teeth andsupport the wafer at its edge. A small peripheral edge portion of eachwafer is simultaneously received within one of the Y-shaped troughs 58(FIG. 6) between adjacent teeth on the stabilizing rod 42, where thewafer is suspended above the bottom 62 of the trough. The carrier may belifted by gripping the end members 20 either manually or by a robot endeffector. The carrier is transported to a cleaning tank, and duringtransport any vibration of the wafers is limited by the teeth 56 on thestabilizing rod 42. The carrier with the wafers is immersed in a liquidmedium, such as a chemical solution, within the cleaning tank andultrasonic waves are generated, typically at the bottom of the tank. Thesonic waves reach all portions of the wafer surfaces, with no blockageof the upper portions because the carrier 10 has a low-slung,minimal-bulk structure. The carrier and wafers are then rinsed withdeionized water and dried with isopropanol vapor. Because the wafers aresupported at point contacts rather than in slits as in some othercarriers of the prior art, there are no locations where fluid collectsand where drying is inhibited. Therefore, the carrier permits evendrying and inhibits formation of water marks or the like on the wafersurfaces.

Thus, the carrier 10 receives and holds a plurality of semiconductorwafers and permits all portions of wafer surfaces to be adequatelycleaned and evenly dried. The carrier prevents wafer damage when thecarrier is moved or vibrated.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results obtained.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean that there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A carrier for receiving and holding a pluralityof semiconductor wafers and permitting surfaces of the wafers to receivemaximum exposure to ultrasonic waves during a wafer cleaning process inwhich the carrier and the wafers are immersed in a liquid medium andultrasonic waves are generated in the liquid medium, the carrier beingadapted to hold wafers having a size defined by a wafer diameter, thecarrier comprising: two vertical sidewalls in spaced relationship; atleast three horizontal supporting rods positioned in spaced relation forsupport of wafers in generally parallel relationship with respect toeach other, each of the supporting rods extending between the sidewalls;a series of spaced apart teeth on at least one of said supporting rods,each adjacent pair of teeth defining a channel therebetween having ashape generally in the form of a “V” to receive a peripheral edgeportion of a wafer for holding the wafer in spaced apart positionrelative to other wafers in the carrier; at least one stabilizing rodextending horizontally between the sidewalls to limit wafer motionrelative to the carrier for steadying of wafers; and a series of spacedapart teeth on the stabilizing rod and troughs formed between each twoadjacent teeth for receiving wafers, each trough being sized and shapedso that the wafer is loosely received therein to permit the wafer tomove in vibration between a nominal position in which the wafer issupported by the supporting rods substantially free of any contact withthe teeth on the stabilizing rod, and a deflected position in which thewafer engages one tooth on the stabilizing rod at a side of the troughto prevent further vibratory deflection of the wafer.
 2. The carrier asset forth in claim 1 wherein the teeth on the stabilizing rod havegenerally vertical edges forming sides of said troughs, the sides ofeach trough being in parallel spaced relationship with a spacing thatexceeds a thickness of a wafer received therein.
 3. The carrier as setforth in claim 2 wherein the sides of each trough are spaced frombetween about 10% and about 20% greater than said thickness of water. 4.The carrier as set forth in claim 1 wherein said horizontal supportingrods are collectively positioned in spaced relationship defining an arcthat corresponds to a segment of a curved peripheral edge on each wafer,the supporting rods being adapted for engaging wafers at points alongthe peripheral edge.
 5. The carrier as set forth in claim 4 whereinthere are at least four supporting rods including two rods having teeth,and two rods that are free of teeth.
 6. A carrier as set forth in claim1 wherein each sidewall has a maximum height generally in a middle ofthe sidewall and tapers in height toward laterally opposite ends of thesidewall.
 7. A carrier as set forth in claim 1 wherein each sidewall issized to have a maximum height that is less than about half of the waferdiameter.
 8. The carrier as set forth in claim 7 wherein each sidewallhas an upper peripheral edge defining a generally convex shape of thesidewall.
 9. The carrier as set forth in claim 8 wherein said maximumheight of each sidewall is between about ¼ and about ½ of the waferdiameter.
 10. The carrier as set forth in claim 9 wherein the twosidewalls are generally aligned and have substantially the same size andshape.
 11. The carrier as set forth in claim 1 wherein the carrier hasat least some peripheral edge surfaces that are tapered in profile sothat liquid will flow off the edge surfaces.
 12. The carrier as setforth in claim 1 wherein a tip of each tooth is blunted thereby toinhibit damage to a wafer that may inadvertently impact the tip.
 13. Acarrier for receiving and holding a plurality of semiconductor wafersand permitting surfaces of the wafers to receive maximum exposure toultrasonic wave irradiation during a wafer cleaning process in which thecarrier and the wafers are immersed in a liquid medium and ultrasonicwaves are generated in the liquid medium, the carrier being adapted tohold wafers having a size defined by a wafer diameter, the carriercomprising: spaced apart sidewalls; at least four horizontal rodscollectively positioned for supporting wafers in generally upright, faceto face position generally parallel to each other, each of the rodsextending between and interconnecting the sidewalls, the rods being inspaced relationship defining an arc that corresponds to a segment of acurved peripheral edge on each wafer, the rods being adapted forengaging wafers at points along the peripheral edge, the rods furtherbeing positioned so that wafers are exposed to said liquid medium andultrasonic waves between said rods; and a series of spaced apart teethon two of said rods, with two of said rods being free of teeth; whereineach adjacent pair of teeth is adapted to receive a peripheral edgeportion of the wafer for holding the wafer in spaced apart positionrelative to other wafers in the carrier, the sidewalls being oriented sothat the carrier is adapted to hold wafers generally parallel to thesidewalls, the sidewalls being sized to have a maximum height that isless than about half of the wafer diameter so that an upper half of eachwafer is exposed to the liquid medium when the carrier holding saidwafers is immersed in the liquid medium, whereby the carrier is adaptedto hold wafers to permit generally unobstructed passage of ultrasonicwaves through the liquid medium to reach all portions of wafer surfaceswith minimal impedance by structure of the carrier, thereby facilitatingeffective cleaning.
 14. The carrier as set forth in claim 13 whereineach sidewall has an upper peripheral edge defining a generally convexshape of the sidewall.
 15. The carrier as set forth in claim 13 whereina tip of each tooth is blunted thereby to inhibit damage to a wafer thatmay inadvertently impact the tip.